Democratic presidential candidate Joe Biden says President Donald Trump is trying to dismantle the limits on the presidency.

The former vice president said Tuesday that in the 2020 election, voters must not only reject Trump's policy agenda and values, but they also must stop Trump's attempt to elevate the presidency above its coequal branches of government.

Biden accused Trump of "breaking down the barriers that constrain his power" and claiming that he has "complete power."

Biden thundered into the microphone: "No, you don't, Donald Trump!" It sparked an eruption of cheers from the 500 people in a building on the Mississippi Valley Fairgrounds in Davenport.

For his part, Trump didn't name-drop Biden at a state party fundraiser Tuesday night, though he mentioned him readily throughout the day.

Iowa's heavily rural economy is anxious about trade disputes he's initiated that have led to retaliatory tariffs on their products, hurting farm income.

At a state Republican Party dinner on Tuesday, Trump urged attendees to call their lawmakers and tell them to vote for a new trade agreement with Canada and Mexico called USMCA, saying, "It's going to be phenomenal for your state."

Trump claims he is "knocking down barriers for American farmers and opening brand-new markets for American agriculture." Regardless, he says the "Democratic Party has never been angrier."

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6:10 p.m.

Former Vice President Joe Biden is preemptively criticizing the first Democratic presidential primary debate as simply an "appearance" because of the short speaking time allowed to candidates.

He told reporters Tuesday in Iowa that because of the crowded field, "we're told we have one minute to respond, one minute to speak."

Biden says he's going to "say why I'm running and what I stand for," and suggested other candidates should do the same, warning them against tossing barbs at one another.

He says doing so would only make it easier for President Donald Trump to win a second term.

Many of Biden's opponents haven't adhered to that strategy, with a number issuing veiled critiques of the front-runner during a weekend political event in Des Moines.

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5 p.m.

President Trump has signed an executive order intended to simplify the regulatory process for genetically engineered agriculture.

The order, signed Tuesday in Iowa, comes as companies are turning to newer genetic engineering techniques that make it easier to tinker with the traits of plants and animals.

Federal agencies didn't immediately have details on the executive order. But the White House said in a statement that the order is intended to help eliminate delays and reduce costs for producers.

Greg Jaffe, biotechnology director at the Center for Science in the Public Interest, says the impact of the order will depend on the details of how it's carried out by federal agencies.

But he says simply deregulating could make people lose confidence in genetically engineered foods.

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4:55 p.m.

President Donald Trump and his 2020 Democratic rival Joe Biden are getting in some digs at each other during separate campaign speeches in Iowa.

During a speech on renewable energy Tuesday, the president compared Biden to his 2016 rival Hillary Clinton, making the claim that both were too focused on him as they campaigned for president.

Trump says, "Then when it came time to vote, they all said, 'You know, she doesn't like Trump very much, but what else does she stand for?' The same thing is happening with Sleepy Joe."

For his part, Biden name-checked the president about a dozen times over the course of two events in Iowa on Tuesday. He told crowd in Mount Pleasant that he finds Trump's interest in him "fascinating."

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4:30 p.m.

President Donald Trump is reminding Iowa voters "I fought very hard for ethanol" as he visits the critical first-in-the-nation caucus state.

Trump says that it was his administration that approved year-round sales of gasoline with higher blends of ethanol, a boon to the top corn-producing state in the nation.

Trump says that using gasoline blends with up to 15 percent alcohol will mean more energy. He says, "What can be wrong with that?"

Critics say the move could hurt millions of consumers whose vehicles and equipment are not compatible with higher-ethanol blended gasoline.

Trump also says his trade policies will help Iowa farmers. Many have faced lower prices as a result of retaliatory tariffs.

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4:25 p.m.

Biden is taking hits from both pro- and anti-abortion activists on the campaign trail in Iowa after reversing his stance on a controversial abortion amendment last week.

At his first event Tuesday, a man and a woman protested Biden's past support for the Hyde Amendment, which blocks federal funding for abortion services. Biden later said he'd like to repeal it.

The woman shouted, "Uterus, mine!"

As the crowd began to shout down the protesters, Biden first joked, "This is not a Trump rally," before asking them to talk to him after the event.

At his second event, a man shouted that Biden "supports the murder of unborn babies, and he wants us to pay for it!"

Biden joked with the crowd that "he followed me in a large bus. ... I thought he was gonna drive me off the road."

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3:20 p.m.

President Donald Trump is getting briefed on Midwestern flood damage as he visits Nebraska prior to participating in political events in Iowa.

March flooding caused at least $3 billion in damage in Nebraska, Iowa, Missouri and Kansas. About two dozen levee systems were breached or overtopped during Missouri River flooding that devastated parts of the states.

More flooding is likely in places protected by levees that were damaged in March because few have been repaired.

In Iowa, 58 of the state's 99 counties are now eligible for public disaster assistance through the Federal Emergency Management Agency.

The Army Corps of Engineers predicts that 50 million acre-feet of water will flow through the reservoirs along the Missouri River this year, the second-highest total ever.

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1:35 p.m.

Democratic presidential candidate Joe Biden is blasting President Trump for his behavior during the D-Day commemoration in Europe last week, including a tweet attacking singer and Broadway star Bette Midler.

During a speech Tuesday to about 100 people in working-class Ottumwa, Iowa, Biden repeated his oft-stated claim that Trump is a "threat to our core values."

Biden, with a look of disbelief on his face, said, "He found time to go after Bette Midler in the middle of the D-Day ceremonies."

Trump called Midler a "Washed up psycho" in a late-night tweet after she apologized for an incorrect statement she made criticizing Trump.

With Trump traveling to Iowa, the former vice president is attacking the president specifically on his economic policies at the outset of a two-day trip, focusing on economically struggling southeastern Iowa.

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1 p.m.

Preparing to hold dueling events in Iowa Tuesday, President Donald Trump is employing schoolyard taunts for his leading Democratic presidential rival, former Vice President Joe Biden.

Trump says of Biden, "I think he's the weakest mentally" of the 2020 field and referred to Biden as "a dummy." Trump addressed reporters from the White House before departing for Iowa to deliver remarks on energy and attend a political fundraiser.

Trump says Biden was wrong to say that China was not a competitor of the U.S., and says that during the Obama administration, China "ate our country alive."

Biden is holding events in the first-in-the-nation caucus state Tuesday, including delivering a speech in which he is expected to call Trump an "existential threat" to the nation.

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8:20 a.m.

The White House says Joe Biden's plan to call President Donald Trump an "existential threat" to the nation is "truly laughable" as the two politicians converge in Iowa on Tuesday.

White House press secretary Sarah Sanders says she's not sure whether Trump will respond to the former vice president, whose prepared remarks criticizing Trump were released ahead of his Iowa speech.

Sanders says, "The idea that he would say that the president poses any type of threat is truly laughable considering he was part of the administration that allowed Russia to interfere in our election."

Sanders says Biden was a key member of an Obama administration that also allowed China to grow and North Korea to test missiles. She says Biden has "got a lot of explaining to do."

Biden plans to use his Iowa visit to criticize Trump's economic policy.

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5:40 a.m.

Democratic presidential candidate Joe Biden plans to use his visit to Iowa to criticize President Donald Trump's economic policy as hurting those very voters who helped elect him.

Biden says in remarks prepared for delivery on Tuesday that Trump "thinks he's being tough" and "it's easy to be tough when someone else is feeling the pain." Biden plans to speak in blue collar Ottumwa, the seat of Wapello County.

Trump was the first Republican to carry the economically struggling county in southeast Iowa since Dwight Eisenhower.

Biden asks in his prepared remarks, "How many sleepless nights do you think Trump has had over what he's doing to America's farmers? Zero."

Trump has attacked Biden regularly and on a recent state visit to Japan echoed North Korean leader Kim Jong Un's description of Biden as "low IQ."

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12:20 a.m.

After months of jabbing from afar, President Donald Trump and Democrat Joe Biden are overlapping Tuesday in Iowa, a state that's critical to their political futures.

The former vice president is in Iowa hoping to hold his party's front-runner role. Trump seeks to shore up support in a key Midwestern state he wrested from Democrats in 2016.

The day could offer a glimpse of a Trump-Biden matchup in this battleground state.

Trump begins his trip in Council Bluffs to speak at Southwest Iowa Renewable Energy, which produces and sells the corn-based fuel additive ethanol. He will later address an Iowa GOP dinner in Des Moines.

Biden is coming two days after nearly 20 Democratic rivals were in Iowa for a state party dinner. Several took veiled shots at his absence.